Following yesterday's explosion at Fukushima Daiichi's unit 2 reactor, a decision was made by a manager on site to evacuate staff working in the area.

But around 50 employees - dubbed the Fukushima 50 - have remained at the site working tirelessly around the clock to avoid possible meltdowns at three reactors at the quake-hit nuclear power plant.

They are attempting to cool down fuel rods at three reactors by injecting seawater into them.

Despite wearing protective clothing, experts say there will be negative effects to their health as a result of the radiation levels.

David Richardson, a professor of epidemiology at the university of North Carolina who has studied the long-term health risks for nuclear plant workers, told the BBC those at Fukushima would receive in an hour the same amount of radiation a US nuclear worker is exposed over an entire career.

"These workers in a few hours are getting fairly high doses I would say by contemporary standards for worker protection and that's likely to pose some risks down the line.

"To my knowledge there's not a good way after exposure of trying to protect somebody from the risks of a subsequent later cancer."

Lee Tin-lap, a toxicologist at a Hong Kong university, told Reuters the current radiation levels would not be immediately dangerous - but there could be long-term effects.

"You are still breathing this into your lungs, and there is passive absorption in the skin, eyes and mouth and we really do not know what long-term impact that would have," he said.

Following the fire at unit 4 yesterday, radiation levels peaked to
levels dangerous for human health.

At 10.22am (2.22pm NZT) a radioactivity monitoring post near the unit 3 reactor showed 400 millisieverts per hour, 400 times the amount an average person is exposed to in a year, the Japan Times reported.

The radiation level was 100 millisieverts per hour near the unit 4 reactor and 30 millisieverts per hour between the unit 2 and 3 reactors.

An official at the Institute of Applied Energy told the Japan Times radiation exposure of 7,000 to 10,000 millisieverts per hour is considered a lethal dose.

"There is no doubt it is an amount that would have (a harmful) effect on the human body."

Without protective gear, exposure to 100 millisieverts per hour can render a man infertile, one Japanese expert told NHK News.

Chief Cabinet Secretary Yukio Edano said yesterday the levels would "no doubt" have a harmful effect on the human body.

"But that is the amount right near the leak. The farther away, it drops."

Radiation readings at the site have since fallen and stabilised, but are still above safe levels.

The situation - which is currently rated a level 4 accident by Japanese officials (Chernobyl was a level 7) - has led to the evacuation of around 200,000 people from nine towns within 20km of the Fukushima Daiichi nuclear power plant and within 10km of the Fukushima Daini plant.

Officials in Fukushima say around 190 people may have been exposed to radiation, and around 230,000 units of iodine have been distributed to evacuation centres in the area around Fukushima Daiichi and Fukushima Daini as a precautionary measure.

The ingestion of stable iodine can help to prevent the accumulation of radioactive iodine in the thyroid.

Residents outside of the evacuation areas have been advised by authorities to stay indoors.